Since the first lunar missions in the 1960's, the world's space agencies have taken interest in the moon as an object of both scientific research and potential commercial development. During the 1980's, several lunar missions were launched by national space agencies and additional lunar missions are planned for the 1990's. In addition, the United States National Aeronautics and Space Administration has long been interested in establishing a lunar base to conduct scientific research and mining activities. Interest in the moon is likely to increase with the advent of the multi-national space station, scheduled for completion in the next decade, making it possible to stage lunar missions from low earth orbit. However, continued interest in the moon and the feasibility of a lunar base will depend, in part, on the agencies' ability to schedule frequent and economical lunar missions.
A typical lunar mission comprises the following steps. Initially, a spacecraft is launched from earth or low earth orbit and sufficient impulse per unit mass, or change in velocity, is provided to place the spacecraft into an earth-to-moon orbit. Generally, this orbit is a substantially elliptic earth-relative orbit having an apogee selected to nearly match the radius of the moon's earth-relative orbit. As the spacecraft approaches the moon, a change in velocity is provided to transfer the spacecraft from the earth-to-moon orbit to a moon-relative orbit. An additional change in velocity may then be provided to transfer the spacecraft from the moon-relative orbit to the moon's surface if a moon landing is planned. When a return trip to the earth is desired, another change in velocity is provided which is sufficient to insert the spacecraft into a moon-to-earth orbit, for example, an orbit similar to the earth-to-moon orbit. Finally, as the spacecraft approaches the earth, a change in velocity is required to transfer the spacecraft from the moon-to-earth orbit to a low earth orbit or an earth return trajectory.
The propellant required at each step above depends on the mass of the spacecraft and the change in velocity required. The spacecraft pays a mass penalty if unnecessary equipment is carried through any of the above steps. Conversely, for example, if the spacecraft could leave unnecessary equipment in orbit around the moon while a rendezvous vehicle touched down on the moon's surface, less maneuvering propellant would be required. Such unnecessary equipment might include earth re-entry shielding and elements of the spacecraft's cryogenics and extended life support systems. As can be understood, an even greater propellant savings would be achieved if such equipment was not transferred from the earth-to-moon orbit to a moon-relative orbit, or if the cryogenics, extended life support, and other systems could remain in orbit for use in future missions.
Accordingly, objectives of the present invention include the following.
Provision of a method for using the moon's gravitational field to return a spacecraft to earth.
Provision of an orbital system which allows a spacecraft to achieve line-of-sight contact with the lunar ecliptic poles with relatively low propellant requirements.
Provision of an orbital system which allows a moon-bound spacecraft to make at least two close lunar approaches and return towards the earth, all orbital transfers being achieved substantially by using the moon's gravitational field.
Provision of an orbital system which allows a spacecraft to make repeated close approaches to both the earth and moon, the system being sustainable with relatively low propellant requirements thereby providing an efficient method for cislunar travel.
Provision of an orbital system which does not require large propellant-supplied changes in velocity, thereby rendering practicable massive spacecraft components, such as solar radiation shielding.
Provision of frequent earth return possibilities from the moon with relatively few spacecraft.
Provision of an orbiting depot for equipment thereby allowing lunar missions to utilize rendezvous vehicles which maneuver efficiently.
Provision of an orbiting depot for equipment thereby allowing rendezvous vehicles to descend into the moon's gravity well with reduced vehicle mass.
Provision of a system for shuttling equipment, personnel, and supplies between the earth and moon.
Other objects and advantages of the present invention will be apparent upon consideration of the following summary and description of the invention.